Motor-operated valves are widely used in various piping systems in plants such as nuclear power plants. To maintain the motor-operated valves in good conditions is very important for the security in the plants. An inspection cycle is determined for each motor-operated valve based on the period and frequency of its operation or data acquired in the past, and each motor-operated valve is subjected to an overhaul periodically to achieve an expected result.
However, such an inspection method involving disassembly requires removal of many components in order to disassemble the motor-operated valve. The disassembly work not only requires skilled workers but also may damage the components. When many motor-operated valves are subjected to such inspection, the amount of work is so large that it is difficult to obtain sufficient skilled workers.
In recent years, an inspection method with which a failure in a motor-operated valve can be detected without disassembling the motor-operated valve is proposed. Such a method includes determining a failure in a motor-operated valve by detecting the time period required for the motor-operated valve to open or close, the torque of the valve driving system, or the wear of a stem nut and so on. One of the most important items to be checked is a torque set value.
Such a motor-operated valve is provided with a torque switch to ensure sufficient opening and closing force and to prevent it from being damaged by excessive driving force during operation. The torque switch is activated by the movement of a worm shaft in its axial direction to turn off the driving energy applied to the driving motor for driving the motor-operated valve.
The diagnosis on whether the operation of the torque switch is appropriate is made by detecting the torque value at the time when the driving energy is turned off (which is referred to as “torque set value”) and comparing it with a reference value.
According to a standard (JEM1446), the actual measured value of the torque set value should be in a range of ±10% of the design value. When the measured value exceeds ±10% of the design value, the motor-operated valve is determined to be abnormal since it cannot provide an appropriate holding force when it is in a close (or open) position.
In the field of motor-operated valve, the following methods are employed as methods for determining the time when the driving energy to the motor is turned off:    (1) By detecting the current flowing through one of the singles phase lines in the power source cable with a clamp meter attached thereto (For example, Research Report of Intelligent Mechatronics Research Division, issued on Sep. 21, 1995 by The Japan Society for Precision Engineering (FIG. 3 in Page 14, Table 2 in Page 15) and JP-A-H1-128561 (FIG. 1)); and    (2) By detecting the current flowing through the power source cable with a clamp meter attached thereto, or by detecting the current in the motor with a magnetism sensor attached thereto (Japanese Patent No. 3,411,980 (JP-A-2002-130531l )).
The method (1) can measure the time when the driving energy is turned off precisely. However, to attach a clamp meter to a single-phase line or the power source cable, the cover of an electrical box has to be removed. In the electrical box, a cable terminal, a torque setting mechanism and so on are located around single-phase lines and the power source cable. Thus, it is difficult to attach the clamp meter when the plant is operating. Also, this method cannot solve the problem of safety and the possibility of damaging other components because it involves the removal of the cover of an electrical box.
In the case of the above method (2), the clamp meter and the magnetism sensor are temporally attached to the power cable and the motor, respectively, a diagnosis can be made even when the plant is operating. However, the driving energy-off time measured by this method includes a large error.
The relation between the driving energy detected by a magnetism sensor attached to a motor and the torque value is shown in FIG. 3.
As can be understood from the graph, when the torque value is rapidly increased, it is increased by about 10% or more within a period corresponding to half a cycle of the driving energy. Since the motor is kept rotating by inertia after the driving energy has been shut off, induction current is kept flowing although it is decreased gradually. Thus, the timing when the driving energy is shut off cannot be read precisely, and torque set value may exceed the permissible limit even when there is a difference of about half a cycle in the reading time. When the timing when the driving energy is turned off is measured with a magnetism sensor attached to the motor, the judgment about when the driving energy is turned off may be different every time it is made or depending on who makes it, resulting in measurement errors. Thus, this method is not practical.
When a clamp meter is attached to a power cable to make measurement without opening an electrical box for the motor-operated valve (during operation), the measurement has to be made from the outside of a power cable tube in which the power cable is housed. When a clamp meter is attached to a power cable tube, since it cannot be attached to a single-phase line from the outside of the power cable tube, it has to clamp all the single-phase lines together. In this case, the clamp meter can measure only a leakage current signal and cannot acquire signal information with a sufficiently high intensity.